The administration of intravenous medical fluids to a patient is well known in the art. Typically, a solution such as saline, glucose or electrolyte contained in a glass or flexible container is fed into a patient's venous system through a conduit such as a polyvinyl chloride (PVC) intravenous (IV) tube which is accessed to the patient by a catheter. Many times, the fluid is infused under the forces of gravity, and the rate of flow is controlled by a roller clamp which is adjusted to restrict the flow lumen of the IV tube until the desired flow rate is obtained.
Flow from the container to the patient also is known to be regulated by means other than a roller clamp. It is becoming more and more common to use an electronically controlled infusion pump. Such infusion pumps include, for example, peristaltic-type pumps and valve-type pumps. Valve-type pumps employ pumping chambers and upstream and downstream valves to sequentially impart the propulsion to the fluid. Peristaltic-type pumps typically include an array of cams angularly spaced from each other which drive cam followers connected to pressure fingers. These elements cooperate to impart linear wave motion on the pressure fingers. This linear wave motion is used to apply force to the IV tube, which imparts the motion to the fluid in the IV tube, thereby propelling the fluid. An alternative type of peristaltic pump employs a plurality of roller members which roll over the IV tube to impart the motion to the fluid in the IV tube. Such infusion pumps include various motors. Examples of such motors include driving motors which drive the pumping hardware and tube loading motors which drive tube loading hardware. Such driving motors can be variable speed motors.
The accurate monitoring of the motors of such infusion pumps with closed loop control systems is desirable in a number of areas. For example, infusion pumps in the art have the potential for mechanical effects on the rotation of the motor during the pumping period. Such mechanical effects can be caused by, for example, temperature variations, variable battery power, motor friction, tubing variances and peak torque requirements. When detected, such mechanical effects can be solved by adjusting power supplied to the motor to compensate for the torque variation. Another area in which accurate monitoring is important is in monitoring the direction and rotation of the motor. Pump motors can move in an abnormal direction as a result of, for example, improper application of the driving signal. When such abnormal operation occurs, it is desirable to signal such abnormal operation so correction can be made.
Use of closed loop control systems which employ positional transducers on the motor shaft to monitor infusion pump motors is known in the art. Such electromechanical position encoder/decoders generally comprise a quadrature shaft encoder whose two outputs sense the presence or absence of an idicia or a flag to indicate motor shaft positional information relative to some starting point. Use of two channels generating quadrature signals allows the direction as well as the distance moved to be monitored and is further known in the art. The resolution of the monitoring information is determined by the number of flags on the encoder wheel. A decoder such as an up-down counter preceded by an appropriate state decoding circuit is used to process the signals from the encoder. The state decoding logic samples the input signals at a rate which ensures that, at the maximum signal frequency, consecutive states are guaranteed to be sampled. The up-down counter output provides positional information relative to the starting position.
Such motor shaft encoder/decoders of the prior art, however, suffer from several drawbacks. The input signals are asynchronous to the decoder clock, and if the state decoder logic samples the input signals at the signal edge, the decoder may enter a metastable state which is indeterminate and may result in incorrect updating of position. While the likelihood of metastability occurring can be reduced to acceptable levels by appropriate digital design techniques known in the art, implementation of these techniques increases the cost of such systems.
Thus, what is needed is a motor shaft encoder/decoder which avoids the issue of metastability during state transitions of the signals from a quadrature shaft encoder. The encoder/decoder also should effectively gather rotational directional information. Additionally, such a device should avoid use of a separate counter or clock. The device should provide these benefits in a cost efficient and effective design.